High-resolution Magnetic Mapping Research Articles

Size-Specific Spin Configurations in Single Iron Nanomagnet: From Flower to Exotic Vortices.

The different spin configurations in the vicinity of the single-domain/vortex transition are reported in isolated magnetic nanoparticles. By combining chemical synthesis, electron holography in a dedicated transmission electron microscope and micromagnetic simulations, we establish the "magnetic configurations vs size" phase diagram of Fe single-crystalline nanocubes. Room temperature high resolution magnetic maps reveal the transition between single-domain and vortex states for Fe nanocubes from 25 to 27 nm, respectively. An intermediate spin configuration consisting of an ⟨111⟩ vortex is for the first time evidenced.

Stokes IQUV magnetic Doppler imaging of Ap stars – II. Next generation magnetic Doppler imaging of α2 CVn★

We present updated magnetic field maps of the chemically peculiar B9p star $\alpha^2$ CVn created using a series of time resolved observations obtained using the high resolution spectropolarimeters ESPaDOnS and Narval. We compare these new magnetic field maps with the original magnetic Doppler imaging maps based on spectra recorded with the MuSiCoS spectropolarimeter and taken a decade earlier. These new maps are inferred from line profiles in all four Stokes parameters using the magnetic Doppler imaging code INVERS10. With the addition of new lines exhibiting Stokes $IQUV$ signatures we have a unique insight into how the derived magnetic surface structure may be affected by the atomic lines chosen for inversion. We report new magnetic maps of $\alpha^2$ CVn created using strong iron lines (directly comparable to the published MuSiCoS maps), weak iron lines and chromium lines, all of which yield a magnetic field structure roughly consistent with that obtained previously. We then derive an updated magnetic structure map for $\alpha^2$ CVn based on the complete sample of Fe and Cr lines, which we believe to produce a more representative model of the magnetic topology of $\alpha^2$ CVn. In agreement with the previous mapping, this new updated magnetic map shows a dipolar-like field which has complex sub-structure which cannot be explained by a simple low order multipolar geometry. Our new maps show that regardless of the atomic line or species choice, the reconstructed magnetic field is consistent with that published previously, suggesting that the reconstructed field is a realistic representation of the magnetic field of $\alpha^2$ CVn. $\alpha^2$ CVn is the first Ap star for which multiple, high resolution magnetic maps have been derived, providing important observational evidence for the stability of both the large and small-scale magnetic field.

Magnetic and ground probing radar measurements for soil pollution mapping in the industrial area of Val Basento (Basilicata Region, Southern Italy): a case study

The joint application of electromagnetic techniques for near-surface exploration is a useful tool for soil pollution monitoring and can also contribute towards describing the spatial distribution of pollutants. The results of a geophysical field survey that was carried out for characterizing the heavy metal and waste disposal soil pollution phenomena in the industrial area of Val Basento (Basilicata region, Southern Italy) are presented here. First, topsoil magnetic susceptibility measurements have been carried out for defining the spatial distribution of superficial pollution phenomena in the investigated area. Second, detailed and integrated measurements based on a high-resolution magnetic mapping and ground probing radar (GPR) profiling have been applied to investigate the subsurface in two industrial areas located in more polluted sites that were identified during the first phase. Our monitoring strategy discloses the way to rapidly define the zone characterized by high pollution levels deriving from chemical industries and traffic emissions and to obtain the way information about the presence of local buried sources of contamination.

Soil anomaly mapping using a caesium magnetometer: Limits in the low magnetic amplitude case

Caesium magnetometers are new tools for soil property mapping with a decimetric resolution [Mathé, V., Lévêque, F., 2003. High resolution magnetic survey for soil monitoring: detection of drainage and soil tillage effects. Earth and Planetary Science Letters 212 (1–2), 241–251]. However, when the magnetic anomalies are only a few nanoteslas (nT), the geologic and pedogenic signal must first be isolated from magnetic disturbances for this method to be useful. This paper investigates the instrumental artifacts and environmental disturbances to adapt the survey protocol to slightly magnetic soils. Among the possible instrumental sources of disturbances listed and quantified, the most significant are: 1) The battery effect upon sensors 2 m away (classic protocol, about ± 0.15 nT) while increasing this distance up to 10 m cancelled it; 2) The noise level of magnetometers and sensors, which, according to tests on two magnetometers and three sensors, rarely and randomly exceeds 0.1 nT, but seems to increase with the electronic component age. Among the environmental disturbances, temporal variations such as diurnal variation or fluctuations linked to the moving of metallic masses play a major role, although the pseudogradient or base-station methods have commonly cancelled them. The efficiency of the latter is strongly dependent on the source nature. However, the ground currents and electromagnetic fields propagating in soils cause more problems. As a first step to better understand such disturbance sources, uncommon magnetic signal variations supposedly due to electromagnetic wave conversions and likely linked to the railway traffic are presented. Based on previous results, an adapted protocol using one magnetometer and two caesium sensors (0.3 and 1.6 m above the surface) is proposed to increase the signal / noise ratio. At first, to maintain an accurate horizontal and vertical location of the sensors, the latter are affixed to a wooden handcart running on plastic rails. Rails adapt to micro-topography, thereby decreasing strongly the soil–sensors distance variations. Anomalies due to topography rarely exceed 0.1 nT. Finally, a method to remove diurnal variations from high-resolution magnetic maps is proposed. Parallel profiles performed successively are adjusted by a cross-profile. Assuming that the temporal variations during each profile are negligible (less than 0.05 nT), this technique, contrary to the pseudogradient, preserves both the decimetric and the metric anomalies (gain of more than 1 nT).

High-resolution magnetic mapping of contaminated sediments in urbanized environments

The remediation of contaminated sediments in industrialized harbors and waterways is a growing environmental concern in North America. More than 100 marine ports and 42 inland harbors in the Great Lakes basins are currently listed by U.S. and Canadian regulatory agencies as having severe habitat and water-quality impairments due to bottom-sediment toxicity. Current approaches to remediation include sediment dredging and removal, capping with clean sediment, and in-situ treatment with oxidants.

Geophysical Investigations of Volcanic Terrains: A Case History from the Gawler Range Volcanic Province, South Australia

Flat-lying magnetic cover rocks, especially when they comprise thick volcanic sequences, present a difficult interpretation problem. Magnetic maps of such regions essentially reflect the exposed volcanic rocks and sources within and beneath the volcanic rocks cannot be easily resolved. Yet such areas are often of economic interest. The Gawler Range Volcanic Province in South Australia represents an example of one such volcanic terrain. The Volcanics are distinctly bimodal in composition: the 'older' units are mafic and the 'younger' units dominantly felsic, with few rocks of intermediate composition. The exposed Volcanics cover approximately 25 000 km2 and drill-hole intersections confirm their existence well beyond the present limits. Magnetite is present in both mafic and felsic volcanic rocks. The province has been identified as a target area for possible epithermal gold mineralisation and volcanogenic-hosted base metal mineralisation. Regional gravity and aeromagnetic data are available over the whole province, while part is covered by a recent high-resolution aeromagnetic and aeroradiometric survey. The Gawler Range Volcanic Province is associated with a large gravity high caused probably by mantle underplating or the emplacement of mantle derivatives prior to the extrusion of the volcanics. This is consistent with isotope data which were previously interpreted to indicate a mantle component for the Volcanics. Two major gravity trends (NE and ESE) intersect within the study area. The ESE-striking trend has been modelled as a deep crustal fault with the southern block down-faulted. These two trends are also reflected in the high-resolution magnetic maps once the high frequencies have been removed from the data. Initial images of the high-resolution aeroradiometric and aeromagnetic data correlated very closely with the outcrop and joint patterns. Areas of radiating fracture patterns may represent volcanic centres. The image of the K:U Ratio reflected minor variations in the otherwise homogeneous geochemistry of the Volcanics. Through image processing and filtering, and combining the interpretation of available geophysical data, an inferred geological map for the area has been suggested. The igneous province is interpreted as extending further eastwards beneath part of the Stuart Shelfing consisting of three well-defined units. The first corresponds to the main Gawler Range Volcanic Province, the second to the south-west gabbroic extension and the third to an eastward extension consisting of a felsic core circled by a mafic ring.

High-resolution magnetic mapping of PR-interval phenomena of normal subjects.

In this experimental study we measured the magnetocardiograms (MCGs) of 17 normals at 42 measuring sites in a plane near the chest. The recordings were made in an unshielded environment with a single channel second-order SOUID gradiometer. From these recordings spatial maps were constructed of the atrial depolarisation and repolarisation. The two series of maps were found to be dipolar, but opposite in sign for 14 of the 17 subjects. The relatively small change in the amplitude and pattern of the atrial repolarisation maps built up out confidence in the method of subtracting a computed mean repolarisation map from the maps obtained during the last 40 ms before the QRS-onset. This method of separating His-Purkinje activity from atrial repolarisation led to distinct dipolar patterns in 9 out of the 17 cases. The direction and location of the associated electric current dipole was in good agreement with what can be expected electrophysiologically for His-Purkinje activation.